1. Field of the Invention
The present invention relates to a discharge control device for a power conversion system.
2. Description of Related Art
There is known a discharge control device for a power conversion system, which is configured to make a short circuit between the positive and negative electrodes of a capacitor connected across the input terminals of an inverter connected to a DC power source at these input terminals, the inverter including a plurality of pairs of a high-side switching element and a low-side switching element connected in series, by turning on the high-side and low-side switching elements of one of the pairs at the same time, to thereby discharge the capacitor for the sake of safety, for example, when an abnormal state occurs. For example, refer to Japanese Patent Application laid-open No. 2009-232620. This discharge control device has a structure in which the gate voltage of each of IGBTs used as the switching elements is reduced when the electrodes of the capacitor are short-circuited compared to when the control apparatus operates in the normal state, in order to prevent the discharge current of the capacitor from becoming excessively large.
Generally, manipulation signals of such an inverter are complementary signals used as on/off commands generated through a PWM process performed based on the results of comparison between a command value received from the outside and a carrier common to the high-side and the low-side switching elements. In more detail, the manipulation signal for the high-side switching element is turned on to make an on command while the command value is larger than the carrier, and the manipulation signal for the low-side switching element is turned off to make an off command while the command value is larger than the carrier. However, the above conventional discharge control device has a problem in that, when the high-side and low-side switching elements corresponding to the U-phase of an electric rotating machine powered by the inverter, for example, are turned on at the same time to discharge the capacitor, there may occur a case in which the high-side switching element or the low-side switching element corresponding to the V-phase or the W-phase is applied with the high voltage of the DC power source. In this case, since the gate voltage of the switching elements used to discharge the capacitor is lower than that of the switching elements not used to discharge the capacitor, a current may flow through the switching elements not used to discharge the capacitor, causing the electric rotating machine to rotate.